customized set box for voronoi component

Hi. Has anyone know if there is any way of making customized set box for Voronoi component. I have tried to do many different ways to sort out but has no success yet. I started with making planar srf from irregular curve( site boundary) and extrude it, hoping that it works as an irregular shaped set box.

I thought it kind of makes sense but irregular shaped set box is not recognized. Does anyone know how to sort out?